A group of Google engineers tasked with looking out for online fraud and ad spam noticed something strange recently: photos of cars for sale in China kept getting flagged -- not by the humans, but by a search engine algorithm.

Scammers were posting images of cars they didn't own, breaking into them and selling them for money. So, how did Google detect the con?

The algorithm saw a pattern of quick buys from new accounts, tied together with larger and more subtle patterns, and deduced something was up. It’s not an airtight system: more than a few valid accounts have had their orders delayed while the team checked them out. But in this case, it was able to reach across continents to suss out a scheme its engineers had never even imagined. Cultural differences could fool the humans, but they couldn’t fool the machine.